1. Field of the Invention
The present invention concerns an apparatus for and a method of electrical-discharge machining a workpiece.
2. Description of the Prior Art
A wide range of electrical-discharge machining apparatuses and methods have long been known. Drawing an electrical-discharge wire through a calibrating die before it is introduced at an erosion point to ensure that its diameter will remain constant is known for example. Annealing, stretching, and/or quenching the tip of the wire before it is threaded through the automatic calibrating die are also known. The intention is to stiffen the tip before introducing it into a reference cavity for example.
The basic components of an electrical-discharge machining apparatus are usually a frame accommodating the drive mechanisms and wire system, a rinsing system, a spark generator, and numerical, specifically computerized numerical, controls.
The tool is a cathode in the form of a length of wire, and the workpiece constitutes an anode. The wire, commencing at a reference cavity or point of origin, carves a cavity into or out of the workpiece.
German OS 3 319 935 A1 describes how a wire in the process of eroding the material entrains air bubbles that prevent it from becoming thoroughly wet. The wire overheats locally and tends to break. To counteract this situation the wire can be brushed before it arrives at the erosion point. It can also or alternatively be sprayed from a nozzle with a liquid, purified water for example, that has been induced to oscillate by an ultrasonic generator. The purpose of the brush, nozzle, and oscillator is to clean the wire. The aforesaid document claims that such cleaning definitely augments the wettability of the moving wire. The wire will attract very little air to its surface. The overall result is a wire that is cooler at the erosion point and will not break as easily.
European Patent 0 173 786 A1 discloses increasing the efficiency of an electrical-discharge machining apparatus by cold drawing the wire to define its diameter before it arrives at the erosion point. Several dies are employed. The same document also suggests a measure for improving cold ductility and reducing breakage in a wire that is at least 50% zinc or cadmium. The wire is heated to at least 600 .degree. C. in a hot box before it enters the dies to coat it with either zinc oxide or cadmium oxide. Coating the wire makes it easier to cold draw and less likely to break, which adds up to more efficient machining.
German Patent 2 440 564 C2 discloses an electrical-discharge machining apparatus with a wire that is straightened and oriented and optionally cold calibrated before it arrives at the erosion point. The wire travels first through a heating section and then through a cooling section before arriving at the erosion point. It is calibrated by dies upstream of the heating section. The wire in one embodiment also travels through a bath of unspecified liquid before it arrives at the erosion point. The overall wire can be directly heated in the heating section by directing an electric current through it. The inner surface of the heating-section wall can be designed to reflect heat. The wire can alternatively be heated inductively, specifically with a high-frequency generator that directs current through a coil around the electrical-discharge wire. The wire is not heated enough to oxidize its surface but just hot enough to straighten and orient it.
Still, there is a constantly increasing demand for an electrical-discharge machining to erode material even more rapidly and precisely and leave a smoother surface.